Our studies in the HIV system have focused on the capacity of anti-DR antibodies to inhibit infection of normal cells in vitro. The absence of detectable effects on virus binding or syncytia formation led to an investigation of antibody effects on macrophages present within T cell cultures. First, methods were established for the removal of macrophages from peripheral blood lymphocytes using gelatin and plasma coated culture flasks. Next, it was shown that infection of T cells in cultures of PHA stimulated peripheral blood lymphocytes exhibited a dependence on the number of macrophages available within cultures, little or no infection occurring in the absence of added macrophages. Finally, the effect of anti-DR mAbs on primary infection of macrophage-depleted T cells was investigated in the presence versus the absence of added macrophages. Results revealed that anti-DR mAbs inhibited T cell infection only in the presence of added macrophages, indicating that the macrophage is probably the target of antibody activity. The virus strain used in these studies is strictly T cell tropic and does not appear to infect cultures of purified peripheral blood monocytes.The role of the macrophage in promoting T cell infection with HIV is not clear although it may be related to secretion of lymphokines that regulate T cell activation.